TFEL devices comprise a laminar stack of thin films deposited on a glass substrate. The thin films include a first transparent electrode layer and an electroluminescent (EL) phosphor structure comprising an electroluminescent phosphor material sandwiched between a pair of insulators. A rear electrode layer completes the laminar stack. In matrix-addressed TFEL panels, the electrodes are driven by row and column drivers, respectively, and light is produced at the points of intersection between the front and rear electrodes due to the creation of a high-intensity electric field.
It has long been a desirable object in the design of such devices to maximize the luminance of the panel while at the same time conserving energy. Considerable power is required for creating electric fields of intensities which are high enough to cause enough light emission for clear viewing. In the past, a considerable amount of effort has been directed toward designing electroluminescent structures which consume less power and yet provide high luminance.
Some approaches have included the use of multiple layer insulators sandwiching the electroluminescent phosphor film. In particular, one approach has been to use a double layer insulator where one of the layers functioned as a carrier injection layer. In theory, a carrier injection layer augments the number of free electrons available at the interface between the EL phosphor layer and the insulating layer. These electrons enhance the electroluminescent properties of the phosphor layer at lower threshhold voltages, and an example of such an approach is shown in "ZnS:Mn Thin Film Electroluminescent Devices Having Doubly-Stacked Insulating Layers," Mita et al., Japanese Journal of Applied Physics, Vol. 26, No. 5, May 1987, pp. L541-L543. According to this article, an electroluminescent layer comprising ZnS:Mn is sandwiched between a doubly-stacked insulating layer comprising Ta.sub.2 O.sub.5 and SiO.sub.2. While in theory this approach has appeared to be attractive, in actual practice, the presence of a carrier injection layer, which presumes an insulating material having relatively low resistivity, degrades the polarization charge field created at the interface between the EL layer and the insulator which is essential for high efficiency operation. Kobayashi, et al. in "Thin Film ZnS:Mn Electroluminescent Device," IEEE Transactions On Electron Devices, Vol. Ed-29, No. 10, October 1982, proposes a germanium semiconductor layer adjacent the ZnS:Mn layer to provide a carrier injection function. This study found, however, that at higher voltages, the luminance of the panel was lower than in devices having conventional structures. Also, breakdown was a problem at voltages close to the saturation value. The one advantage of the germanium layer was that it was black, thereby providing an increase in contrast under high ambient light conditions. Another stacked insulator device is shown in the paper entitled "Stacked Insulator Structure Thin Film Electroluminescent Display Devices", Ohwaki, et al., Abstract No. 1222, Electrochemical Society Meeting, Honolulu, HI, October 1987. This paper discloses the use of a stacked insulator structure in a TFEL laminate comprising Al/SiO.sub.2 /Ta.sub.2 O.sub.5 /SiO.sub.2 /ZnS;Mn/Ta.sub.2 O.sub.5 /SiO.sub.2 /ITO/glass. The silicone dioxide functions as a carrier injection layer and accordingly has a low resistivity. This device proved not to be as efficient as a conventional TFEL device because it required a higher threshhold voltage for luminescence.
The use of silicon oxynitride (SiON) as a layer in a doubly-stacked dielectric has been proposed in Mizukami et al., U.S. Pat. No. 4,188,565. According to this reference, the function of the SiON is to bond a second dielectric layer, which may be SiO.sub.2 or Ta.sub.2 O.sub.5 on the front, and Al.sub.2 O.sub.3 or Ta.sub.2 O.sub.5 on the back, to the EL layer. The SiON is placed against the ZnS:Mn film on both sides which may cause degradation of the polarization charge and lower the luminance of the panel.
The aforementioned references make it clear that the double-stacked insulator structure has serious deficiencies where the insulator-EL interface is designed as a carrier injection layer and includes a material of relatively low resistivity.